The present invention relates to (i) a superresolution scanning optical device, a laser scanning microscope, a bar-code scanner and a laser printer in each of which an object is scanned by a focused beam to optically process the information, (ii) an image forming optical device such as a disc device which has no scanning means but uses a semicoherent illumination, (iii) a stepper optical device used in a semiconductor producing process, and (iv) a superresolution light source device and a superresolution filter to be used in any of the optical devices above-mentioned.
The superresolution scanning optical device has scanning means for scanning a line or a space with a coherent beam focused in the form of a fine spot, and converting means for photoelectrically converting the light intensity of the scanned beams. In a scanning and photoelectrically converting optical system, there are proposed a variety of arrangements for effectively obtaining a fine spot equal to or smaller than the diffraction limit.
FIG. 19 schematically shows a conventional optical pickup head H proposed for effecting superresolution in an optical disk system ["High Density Optical Recording by Superresolution", Y. Yamanaka, Y. Hirose and K. Kubota, Proc. Int. Symp. on Optical Memory, 1989. Jap. J. of Appl. Phys., Vol. 28 (1989) supplement 28-3, pp. 197-200].
As shown in FIG. 19, a coherent beam emitted from a light emitting point 10a from a coherent light source 10 comprising a semiconductor laser light source, passes through a collimate lens 12, thus causing the coherent beam to be converted into parallel beam portions, each of which is then divided into two portions by a double loam prism 14. These beam portions penetrate a first beam splitter 16 and a second beam splitter 18, and are then focused in the form of a superresolution spot on a magneto-optical medium 22 by an objective lens 20.
In a returning path, beam portions as divided and reflected by the second beam splitter 18, transmit a condenser lens 24 and are intercepted at the side lobes thereof by a slit 26. Then, the beam portions transmit a condenser lens 28 and a Wollaston polarizing prism 30, and reach a first photodetector 32, where the beam portions are photoelectrically converted and read in terms of an electrical signal. Beam portions as divided and passed through the second beam splitter 18, penetrate a condenser lens 34, and each beam portion is further divided into two portions by a third beam splitter 36. These two portions respectively reach a second photodetector 38 and a third photodetector 40, where servo signals for focusing and tracking are detected.
In FIG. 19, a knife edge 42 is disposed for cutting an edge of a beam portion which has penetrated the third beam splitter 36, and an actuator 43 is disposed for driving the objective lens 20.
FIG. 20 shows the principle of superresolution used in the optical pickup head H shown in FIG. 19. More specifically, two beams emitted from the double loam prism 14 in FIG. 19 are equivalent to beams diffracted from two openings 44a, 44b formed in a light intercepting plate 44 in FIG. 20, and present an intensity distribution I(x) shown by a solid line on an image forming plate 46 (X-axis). A dotted line in FIG. 20 shows an intensity distribution I.sub.0 (x) which would be obtained when either the opening 44a or 44b is present alone. It is well known that, since diffraction images from the openings 44a, 44b interfere with each other, there is formed an image of intensity distribution I(x) as shown by the solid line. In this case, the shape of each of the openings 44a, 44b (corresponding to the shape of the light emitting face of the Wollaston polarizing prism 30 in FIG. 19) may be in the form of a slit or a ring.
In reading and scanning in an optical disc device, a signal is to be detected at a photoelectrically converting face. Accordingly, only filtering using a slit is practically available, and there is not used a two-dimensional superresolution effect produced by the use of an annular opening. To restrain a cross talk due to two side lobes (X=-X.sub.1, X=X.sub.1) appearing on the image forming face 46 in FIG. 20, the slit 26 as shown in FIG. 19 is disposed. As to such a superresolution optical system, a variety of examples are proposed also in the field of a scanning optical microscope. In such an optical system, slit-like or annular openings are formed in a face equivalent to the opening face of the objective lens 20, and a predetermined phase and a predetermined amplitude transmissivity are given to each of the areas to obtain a fine spot smaller than of the diffraction limit determined by the maximum opening of the objective lens 20.
When slit-like or annular openings are formed in a face equivalent to the opening face of the objective lens 20 as above-mentioned, superresolution smaller than the diffraction limit can be obtained with the side lobes restrained to a certain extent. However, such an arrangement presents the following problems. That is, since the amount of light reaching the image forming face is remarkably decreased, the amount of light in the main lobe is decreased. Since there are formed the openings for restraining the side lobes, the optical system is required to be adjusted with precision. Since extra light paths are required, the optical system is complicated in arrangement.
In view of the foregoing, the present invention is proposed with the object of providing a simple optical system capable of obtaining superresolution smaller than of the diffraction limit without the amount of light in the main lobe remarkably decreased.